Excessive borehole deviation away from the planned plane in exploratory diamond drill holes is a costly problem for the exploration company. Targets are missed and in some cases re-drilling has to be carried out or wedging of the hole must be done to bring the inclination back on target or close to it. Wedging is also an expensive undertaking.
To minimize the deviation problem, numerous attempts have been made to stabilize the lower part of the drill string, namely, the core barrel and reaming shell. The latter is increased in length with an added diamond and carbide set sintered powder ring fused to the upper part of a blank to maximize flexing of the tool joint between the shell and outer tube. Next the outer tube is stabilized similarly to the shell by fusing three such rings at three foot spacings on the exterior of the outer tube. Similar rings are also employed on the locking coupling.
The other method is to use outer tube material similar in outside diameter to that of the carbide rings and to machine three angularly spaced flats approximately 1/2" wide full length on the exterior of the outer tube. The flats are for the purpose of allowing drilling fluid return from the coring bit. Both of these methods have their shortcomings.
The carbide ring concept requires that the reaming shell outer tube and locking coupling be handled with extreme care in terms of providing for non-contact with pipe wrenches, rod holders and chucks; otherwise the rings will crack and if this goes unnoticed the rings will come off in the drill hole while drilling with serious consequences. Even when using extreme care these rings are susceptible to damage from extreme thrust on the outer tube when penetrating hard rock formations. The outer tube tends to flex or bend, especially at the centre ring, which results in hair line cracks in the rings.
The use of the oversize stabilized outer tube with three angularly spaced flats machined full length on its surface is one alternative to the carbide ring type noted above. The fact that it provides minimal fluid passage area over it's full length (e.g. about 10 feet) is a drawback because the rate of penetration must be controlled to allow the drill bit cuttings to free flow past this lengthy flow restriction. This outer tube is also subject to premature wear since the material used in its make up is standard AISI-SAE 1035 material. Another problem that arises with this version is that the bore hole must be clear of cuttings, sand and cave-in material; otherwise this outer tube can become stuck in the hole because of the restricted overall tolerances.